To signal to the recipient that content negotiation has been used to determine
the best available representation for a given request, the server
must include a Vary header. This tells the
recipient which request headers have been used to determine the
representation that is used. So an answer may be generated like this:

The header of a very cool page may greet the user with something like
this:

Hallo Harri, Dein NutScrape versteht zwar PNG aber leider kein GZIP.

However, this header has the side effect of being expensive for a
caching proxy. As of this writing, Squid (Version 2.3.STABLE4) does
not cache resources that come with a Vary header
at all. So without a clever workaround, the Squid accelerator is of
no use for these documents.